1. Field of the Invention
This invention relates to an optical memory medium and a method for preparing the same, particularly a grooved optical memory medium such as flexible optical disc, etc. and a method for preparation thereof.
Further, the present invention relates to a thin type disc and a method for preparing the same, particularly a thin type high density information recording disc such as flexible video disc, etc., or a grooved optical information recording substrate and a method for preparation thereof.
2. Related Background Art
Preparation of a thin type optical memory medium of the prior art has been practiced by first molding a substrate of a thermoplastic resin such as polymethyl methacrylate, polycarbonate, etc., by injection molding or compression molding into the state in which an uneven guide groove is provided on the surface, and then laminating an optical recording layer on the substrate.
On the other hand, a high density information recording disc has been prepared by molding a thermoplastic resin such as polymethyl methacrylate, polycarbonate, etc., by injection molding or compression molding into the state in which an unevenness for signals is provided on the surface, followed by cooling and solidification.
According to such methods, it has been difficult to make the medium of a thin type, and an optical memory medium with a thickness of 1.0 mm or less could be prepared only with substantial difficulty. This has been primarily because of the fact that, as shown in FIG. 16, the substrate can only could be molded with difficulty for such problems as strength, etc., unless the thickness of the base portion 41 beneath the uneven guide groove portion 40 is made considerably thicker relative thereto.